Help! Earlier this summer, I took my 1998 ML320 to an shop that specializes in automobile air conditoning. They put in freon and dye, and later reported to me that my expansion valve needed to be replaced. Since the freon was added, I've been getting ice cold air from the air conditioner. Today, I have the car in the shop for another problem and mentioned what I had been told by the other shop. They've just called to tell me that my compressor is bad. Is is possible for an a/c compressor to be bad and still put out cold air?
Any help or comments on this would be most appreciated.

Is it making any noise?

If you hear a clacking when it's running, or the clutch is staying engaged when it should be disengaged, then yes it's possible that it's going bad even though the air is still cold. Another possibility is that, when they changed the expansion valve, they noticed debris being thrown off by the compressor.

Any of those situations would require that it be replaced, even though the air is still cold.

But if it's not making any noise, and if it is engaging/disengaging properly, then I would be VERY suspicious, and at the very least require them to thoroughly explain their diagnosis, and then report back here.